Well being a buff & not a real/static mana regen property I would guess not. Some stack but only if the 2nd is greater then the 1st I have always seen (Example drink an Agility then a Gr Agility, you get the 2nd buff but only it's difference added on, not it's full on top of the reg agility). But it's a guess on the buff as I said. It IMO, would be like trying to stack 2 slayer properties on & thinking you would get 2x the damage over having 1 slayer property. Toss it on UO Hall Kal & see if a Dev answers.
This is directly off UO's Item Property page on Straight mana regen:
Regeneration Effects:
Hit Point Regeneration - Increases the wearer's natural Hit Point regeneration rate. Capped at 18.
Mana Regeneration - Increases the wearer's natural Mana regeneration rate. Subject to diminishing returns - the difference between 0 MR and 3 MR is substantially greater than the difference between 15 MR and 18 MR. No hard cap.
Stamina Regeneration - Increases the wearer's natural Stamina regeneration rate. Capped at 24.
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